It is genuinely just because the image of the skinny woman is held up as the goal and beautiful and women who haven't looked into lifting won't have been exposed to the pictures of beautiful women who lift. It's as simple as that.
If all you go to the gym for is some cardio, you won't know much of anything about the fact that women can and do lift. It's a pretty sexist state of affairs that women with good functional strength (and therefore some definition in their arms etc) are generally seen as no longer looking 'feminine'. A friend at uni who did muay thai even said that any hint of ab muscle was ugly. How she managed to avoid gaining any muscle while doing muay thai I have no idea. Maybe she was a massive slacker or maybe she starved herself. I do recall only ever seeing her eat toast and yoghurts. I was made to feel like a weirdo for liking the (very slight) ab definition on Sarah Jessica Parker in an episode of Sex and the City (yes my uni experience was ****ing dreadful, this is the kind of **** that was always on TV in my house).